panel-header-mod.css

/*
Panel Header mod
https://forum.vivaldi.net/topic/18030/panel-header-modification
Hides search and toolbar icons in panel, shows them when hovering header. – Useful for small displays, but looks neat too. ^^
*/

.panel header {
    padding-top: 9px;
}
.panel header h1 {
    margin-bottom: 5px;
}
.panel.panel-bookmarks header:hover > .toolbar.icons, #notes-panel.panel header:hover > .toolbar.icons {
    max-height: 68px;
    margin-bottom: 0px;
    opacity: 1;
}
.panel.panel-bookmarks header:hover h1, #notes-panel.panel header:hover > h1 {
    color: var(--colorHighlightBg);
}
.panel.panel-bookmarks .toolbar.icons, #notes-panel.panel .toolbar.icons {
    max-height: 0px;
    margin-bottom: -6px;
    opacity: 0;
    transition: max-height 0.5s ease-out, margin 0.5s ease-out, opacity 0.5s ease-out;
}